Conventionally, as described in Japanese Laid Open Patent No. 10-298733, when material whose main component is iron is thermal-sprayed on a cylinder bore of an aluminum cylinder block formed by casting, flux (fluoride salt treatment) is coated on the cylinder bore and dehydration thereof is carried out by heating after cleaning, prior to carrying out a thermal spray process after the casting process, in order to increase the adhesiveness between the material and an inner surface of the cylinder bore (antistripping). Further, a nickel alloy that has high adhesiveness to iron and aluminum is thermal-sprayed on the cylinder bore (bond-coating), and then a material whose main component is iron is thermal-sprayed. In such a conventional process, problems occur due to the processing time required for the pre-thermal spray process and the capital investment therefor (from the cleaning to bond-coating).
When a coating is formed by a thermal spray using a material whose main component is an iron on the internal surface of the cylinder bore of a liner-less aluminum cylinder block, which affects reduction of the weight and the exhaust management of a vehicle engine, the internal surface of the cylinder bore may be required to be rough as a preprocess in order to increase adhesiveness of the thermal spray coating.
The adhesiveness thereof is increased by forming the rough surface since the contact area between the inner surface of the aluminum cylinder block bore and the thermal spray film is increased. That is, if adhesiveness per unit area is the same, the adhesiveness increases as the contact area becomes larger, thereby increasing the adhesiveness therebetween and causing an anchor effect.
For example, Japanese Laid Open Patent No. H11-320414 discloses a shot blast process as a method for forming a rough surface. With this method, shot blast particles may sometimes remain on the processed surface. If the thermal spray coating is formed on the base material under this condition, it becomes unstable in that the residue is exposed on the base material surface or adhesiveness of the thermal spray coating is decreased. As a result, reliability is reduced.
As a method for solving such problems, Japanese Laid Open Patents Nos. H10-77807 and 2002-155350 disclose methods for forming a rough surface on the base material by using a cutting tool.
As disclosed in these patents, since one or more spiral grooves are formed on the surface of the base material by using a cutting tool, the adhesiveness of the thermal spray coating later formed is weak against forces applied in the direction of the spiral grooves. Thus, the roughening of the base material surface is not sufficient and overall sufficient adhesiveness of the thermal spray coating cannot be secured, so that the reliability of the thermal spray coating is reduced.
In the present method, the adhesiveness of the thermal spray coating on the internal surface of the cylinder is improved by forming sufficient roughness on the internal surface of the cylinder.
Specifically, in a thermal preprocessing method for forming a rough surface on the internal surface of the cylinder before a thermal spray coating is formed thereon, the inner surface is sufficiently roughened so as to have an uneven shape on the internal surface of the cylinder by pressing of a processing roller having unevenness on its external periphery surface while it rotates and moves. Thereby, blast particle residue on the internal surface of the cylinder otherwise formed during, for example, a shot blast process, is precluded and sufficient roughening of the internal surface of the cylinder is carried out so that the adhesiveness of the thermal spray coating can be improved. The process can include applying nickel material to the uneven outer surface of the processing roller, and thereafter, bringing the outer surface in contact with the cylinder internal surface, and rotating the processing roller relative to the cylinder internal surface while pressing against the cylinder internal surface to roughen, and at the same time, detaching at least some of the nickel material from the uneven outer surface and attaching at least a portion of the detached nickel material to the cylinder internal surface.